Sensors are used in control systems of internal combustion engines and other combustion devices to measure operating parameters and constituents of a resulting feedstream. Sensor information is provided to a controller that can control an incoming feedstream or trigger an alarm based upon the measured parameter or constituent. For example, an exhaust gas sensor in a control system of an internal combustion engine is used to measure the parameter of air/fuel ratio. An engine controller can then use the air/fuel ratio information to control the feedstream that flows through the engine and into an aftertreatment system, such as a catalytic converter. A properly controlled gas feedstream is important for complete operation of the exhaust aftertreatment system during light-off and steady-state warmed-up operation of the control system.
A control system must have accurate, timely feedback from the feedstream to effectively control a device such as an engine. Optimal performance of an exhaust aftertreatment system relies upon a controlled, predictable feedstream. Precise control of the exhaust gas feedstream is becoming more important with the implementation of new engine technologies, including direct injection fuel injection systems and lean-burn control systems.
A sensor takes a certain amount of time to warm up and become fully operational. The amount of time to full operation is affected by the power delivered to a heating element of the sensor and the heat transferred between the sensor by the feedstream and the surrounding environment, including any heat transferred from a mounting structure for the sensor.
The ability to maintain the sensor at a target operating temperature leads to more precise output of the sensor in systems wherein the sensor output is dependent upon the operating temperature. A specific example of an interaction between the operating temperature and measurement ability of a sensor is a zirconium-oxide exhaust gas sensor that is used for internal combustion engine control and diagnostics. The output of the sensor varies as a function of the sensor's operating temperature when measuring in a rich air/fuel region. The voltage output of the zirconium-oxide exhaust gas sensor is a function of the partial pressure of oxygen in the feedstream compared to a reference value, and the operating temperature of the zirconium-oxide cell. This has been described using the Nernst equation, which is a governing equation for a zirconium-oxide exhaust gas sensor:Sensor Output, VSEN(volts)=K*TS*Ln([P(O2)REF]/[P(O2)EXHAUST])                wherein: K=R/(4F)        R=Universal Gas Constant (8.315 J/mole-K)        F=Faraday Constant (96.485 Coulomb/gmole equiv)        TS=Operating Temperature (K) of the Sensor        P(O2)REF=partial pressure of Oxygen, in a reference cell        P(O2)EXHAUST=partial pressure of Oxygen, in the monitored feedstream.        
As can be seen, the operating temperature TS of the sensor directly influences the sensor output, VSEN. A control system can rely more completely on the output of the sensor as a measure of the partial oxygen pressure when the sensor is maintained at a specific temperature. This permits more precise control of the system using the sensor. In the case of the zirconium-oxide oxygen sensor, control of the operating temperature of the sensor which also allows a range of the output of the sensor to be linearized, leading to more precise measurement and control of exhaust gas air/fuel ratio.
The prior art has sought to improve the accuracy, in terms of measurement repeatability, of a gas sensor by adding a heating element to the sensor. The prior art has also sought to control an amount of power delivered to a heating element of a gas sensor so the heating element operates at a specific temperature. It is inferred that the sensor element operates above a minimum temperature, under known conditions. It accomplishes the control of power to the heating element by using basic and auxiliary electric power and relying upon a measure of engine coolant temperature for feedback. This control is primarily focused upon maintaining sensor temperature above a certain level when the system is in a warmed-up operating condition. The prior art has also controls the heating element by measuring an internal resistance of the heating element before and during sensor operation, and controlling the power delivered to the heating element based upon a ratio of the two measured resistances. This strategy heats a sensor to a predetermined temperature using information from the heating element as feedback. The prior art does not control temperature and operation of the sensor based upon any external effects, including heat transfer from the feedstream environment and the sensor mounting structure. The prior art also does not use a physical model to assist in determining the sensor temperature.
Therefore, there is a need to improve the measurement accuracy, repeatability and durability of a sensor by compensating for any effect on operating conditions due to changes in the sensor environment. There is a need to compensate for any effect on sensor temperature caused by changes in the gas feedstream or due to heat transfer between the sensor and a mounting structure for the sensor. There is also a need to determine and control heat energy transfer to a sensor during sensor warm-up.